Mode uniformity signaling for intra-coding

ABSTRACT

Techniques for efficiently signaling one or more prediction modes for blocks in a macroblock. A mode uniformity indicator is provided for each macroblock to indicate whether all blocks in the macroblock are to be predicted using the same prediction mode. In an embodiment, an encoder signals a mode uniformity indicator and a prediction mode. In another embodiment, a decoder receives a mode uniformity indicator and a prediction mode, and predicts at least two blocks of the macroblock using the prediction mode if the mode uniformity indicator is true.

CLAIM OF PRIORITY

This patent application is based on and claims priority to U.S. patent application Ser. No. 60/912,364, filed Apr. 17, 2007, and is a co-pending application of Attorney Docket No. 071347U1, [filed concurrently] entitled Directional Transforms For Intra-Coding, and Attorney Docket No. 071347U2, [filed concurrently] entitled Pixel-By-Pixel Weighting For Intra-Frame Coding, all of which can be assigned to the assignee of the present invention, the contents of which are hereby expressly incorporated by reference herein.

TECHNICAL FIELD

The disclosure relates to digital video processing and, more particularly, to techniques for intra-frame video encoding and decoding.

BACKGROUND

In video encoding, a frame of a video sequence may be partitioned into rectangular regions or blocks. A video block may be encoded in Intra-mode (I-mode) or Inter-mode (P-mode).

FIG. 1 shows a diagram of a prior art video encoder for the I-mode. An encoder may be configured to partition a frame into a plurality of blocks and encode each of the blocks separately. As an example, the encoder may partition the frame into a plurality of 16×16 “macroblocks” that include sixteen rows of pixels and sixteen columns of pixels. Macroblocks may comprise a grouping of sub-partition blocks (referred to herein as “blocks”). As an example, a 16×16 macroblock may contain sixteen 4×4 blocks, or other size sub-partition blocks.

In FIG. 1, a spatial predictor 102 forms a predicted block 103 from video block 100 by using pixels from neighboring blocks in the same frame. The neighboring blocks used for prediction may be specified by a prediction mode 101. A summer 104 computes the prediction error 106, i.e., the difference between the image block 100 and the predicted block 103. Transform module 108 projects the prediction error 106 onto a set of basis or transform functions. In typical implementations, the transform functions can be derived from the discrete cosine transform (DCT), Karhunen-Loeve Transform (KLT), or any other transforms. For example, a set of transform functions can be expressed as {f₀, f₁, f₂, . . . , f_(N)}, where each f_(n) denotes an individual transform function.

The transform module 108 outputs a set of transform coefficients 110 corresponding to the weights assigned to each of the transform functions. For example, a set of coefficients {c₀, C₁, C₂, . . . , C_(N)} may be computed, corresponding to the set of transform functions {f₀, f₁, f₂, . . . , f_(N)}. The transform coefficients 110 are subsequently quantized by quantizer 112 to produce quantized transform coefficients 114. The quantized coefficients 114 and prediction mode 101 may be transmitted to the decoder.

FIG. 1A depicts a video decoder for the I-mode. In FIG. 1A, quantized coefficients 1000 are provided by the encoder to the decoder, and supplied to the inverse transform module 1004. The inverse transform module 1004 reconstructs the prediction error 1003 based on the coefficients 1000 and the fixed set of transform functions, e.g., {f₀, f₁, f₂, . . . , f_(N)}. The prediction mode 1002 is supplied to the inverse spatial prediction module 1006, which generates a predicted block 1007 based on pixel values of already decoded neighboring blocks. The predicted block 1007 is combined with the prediction error 1003 to generate the reconstructed block 1010. The difference between the reconstructed block 1010 and the original block 100 in FIG. 1 is known as the reconstruction error.

An example of a spatial predictor 102 in FIG. 1 is herein described with reference to section 8.3.1 of ITU-T Recommendation H.264, published by ITU—Telecommunication Standardization Sector in March 2005 (hereinafter “H.264-2005”). In H.264-2005, a coder offers 9 prediction modes for prediction of 4×4 blocks, including DC prediction (Mode 2) and 8 directional modes, labeled 0 through 8, as shown in FIG. 2. Each prediction mode specifies a set of neighboring pixels for encoding each pixel, as illustrated in FIG. 3. In FIG. 3, the pixels from a to p are to be encoded, and neighboring pixels A to L and X are used for predicting the pixels a to p. If, for example, Mode 0 is selected, then pixels a, e, i and m are predicted by setting them equal to pixel A, and pixels b, f, j and n are predicted by setting them equal to pixel B, etc. Similarly, if Mode 1 is selected, pixels a, b, c and d are predicted by setting them equal to pixel I, and pixels e, f, g and h are predicted by setting them equal to pixel J, etc. Thus, Mode 0 is a predictor in the vertical direction; and Mode 1 is a predictor in the horizontal direction.

It has been noted that oftentimes a 16×16 macroblock contains 4×4 blocks all encoded using the same prediction mode. It would be desirable to provide an efficient way to signal to a decoder that all blocks in a macroblock are encoded using the same prediction mode.

SUMMARY

An aspect of the present disclosure provides a method for encoding a macroblock, the macroblock comprising a plurality of blocks, each block comprising a set of pixel values, the method comprising for each block, selecting a prediction mode for predicting pixels in the block based on neighboring pixels; and providing a mode uniformity indicator for the macroblock, the mode uniformity indicator indicating whether a plurality of blocks in the macroblock have the same selected prediction mode.

Another aspect of the present disclosure provides a method for predicting a macroblock, the macroblock comprising a plurality of blocks, each block comprising a set of pixel values, the method comprising receiving a prediction mode and a mode uniformity indicator for the macroblock; and if the mode uniformity indicator is true, predicting at least two blocks of the macroblock according to the received prediction mode.

Another aspect of the present disclosure provides an apparatus for generating an encoded macroblock, the macroblock comprising a plurality of blocks, each block comprising a set of pixel values, the encoded macroblock comprising a prediction mode corresponding to at least one block in the macroblock, the prediction mode specifying combinations of neighboring pixels used to predict pixels in the block; and a mode uniformity indicator indicating whether a plurality of blocks in the macroblock have the same prediction mode.

Yet another aspect of the present disclosure provides an apparatus for predicting a macroblock, the macroblock comprising a plurality of blocks, each block comprising a set of pixel values, the apparatus receiving a prediction mode and a mode uniformity indicator for the macroblock, wherein if the mode uniformity indictor is true, the apparatus predicts at least two blocks of the macroblock according to the received prediction mode.

Yet another aspect of the present disclosure provides a computer program product for predicting a macroblock, the macroblock comprising a plurality of blocks, each block comprising a set of pixel values, the product comprising computer-readable medium comprising code for causing a computer to receive a prediction mode and a mode uniformity indicator for the macroblock; code for causing a computer to, if the mode uniformity indicator is true, predict at least two blocks of the macroblock according to the received prediction mode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a diagram of a prior art video encoder for the I-mode.

FIG. 1A depicts a prior art video decoder for the I-mode.

FIG. 2 shows prediction modes described in H.264-2005.

FIG. 3 illustrates pixel prediction using prediction modes.

DETAILED DESCRIPTION

Disclosed herein are techniques to efficiently signal the prediction modes used to encode blocks in a macroblock.

In H.264-2005, a prediction mode is derived for each 4×4 block in a 16×16 macroblock. The prediction mode is signaled from encoder to decoder for each block, i.e., 16 modes per macroblock.

According to the present disclosure, a “mode uniformity” indicator can be signaled on the macroblock level to indicate whether all 4×4 blocks within the macroblock are using the same prediction mode. In an embodiment, the mode uniformity indicator is a bit. If the bit is true, the prediction mode is signaled only once for all the 4×4 blocks in a given macroblock. In an embodiment, the prediction mode is signaled only for the upper left 4×4 block. If the bit is false, the prediction mode is signaled for each 4×4 block, as is prescribed by H.264-2005.

Note the mode uniformity indicator may also be defined to signal mode uniformity for any subset of the blocks in a macroblock. Accordingly, multiple mode uniformity indicators may be provided for a single macroblock for flexibility. For example, a first mode uniformity indicator may be provided for eight 4×4 blocks in the top half of the macroblock, and a second mode uniformity indicator may be provided for eight 4×4 blocks in a bottom half of the macroblock.

Note the techniques disclosed need not be applied to macroblocks having 16×16 pixels, and/or blocks having 4×4 pixels. Other combinations of macroblock and block size may also adopt the signaling described.

Based on the teachings described herein, it should be apparent that an aspect disclosed herein may be implemented independently of any other aspects and that two or more of these aspects may be combined in various ways. The techniques described herein may be implemented in hardware, software, firmware, or any combination thereof. If implemented in hardware, the techniques may be realized using digital hardware, analog hardware or a combination thereof. If implemented in software, the techniques may be realized at least in part by a computer-program product that includes a computer readable medium on which one or more instructions or code is stored.

By way of example, and not limitation, such computer-readable media can comprise RAM, such as synchronous dynamic random access memory (SDRAM), read-only memory (ROM), non-volatile random access memory (NVRAM), ROM, electrically erasable programmable read-only memory (EEPROM), erasable programmable read-only memory (EPROM), FLASH memory, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other tangible medium that can be used to carry or store desired program code in the form of instructions or data structures and that can be accessed by a computer.

The instructions or code associated with a computer-readable medium of the computer program product may be executed by a computer, e.g., by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, ASICs, FPGAs, or other equivalent integrated or discrete logic circuitry.

A number of aspects and examples have been described. However, various modifications to these examples are possible, and the principles presented herein may be applied to other aspects as well. These and other aspects are within the scope of the following claims. 

1. A method for encoding a macroblock, the macroblock comprising a plurality of blocks, each block comprising a set of pixel values, the method comprising: for each block, selecting a prediction mode for predicting pixels in the block based on neighboring pixels; and providing a mode uniformity indicator for the macroblock, the mode uniformity indicator indicating whether multiple blocks in the macroblock have the same selected prediction mode.
 2. The method of claim 1, the mode uniformity indicator indicating whether all blocks in the macroblock have the same selected prediction mode.
 3. The method of claim 1, wherein the mode uniformity indicator is a mode uniformity bit.
 4. The method of claim 1, each block comprising 4×4 pixels, the macroblock comprising 16×16 pixels.
 5. A method for predicting a macroblock, the macroblock comprising a plurality of blocks, each block comprising a set of pixel values, the method comprising: receiving a prediction mode and a mode uniformity indicator for the macroblock; and if the mode uniformity indicator is true, predicting at least two blocks of the macroblock according to the received prediction mode.
 6. The method of claim 5, further comprising predicting all blocks of the macroblock according to the received prediction mode if the mode uniformity indicator is true.
 7. The method of claim 5, the mode uniformity indicator being a mode uniformity bit.
 8. The method of claim 5, each block comprising 4×4 pixels, the macroblock comprising 16×16 pixels.
 9. An apparatus for generating an encoded macroblock, the macroblock comprising a plurality of blocks, each block comprising a set of pixel values, the encoded macroblock comprising: a prediction mode corresponding to at least one block in the macroblock, the prediction mode specifying combinations of neighboring pixels used to predict pixels in the block; and a mode uniformity indicator indicating whether a plurality of blocks in the macroblock have the same prediction mode.
 10. The apparatus of claim 9, the mode uniformity indicator indicating whether all blocks in the macroblock have the same prediction mode.
 11. The apparatus of claim 9, wherein the mode uniformity indicator is a mode uniformity bit.
 12. The apparatus of claim 9, each block comprising 4×4 pixels, the macroblock comprising 16×16 pixels.
 13. The apparatus of claim 9, wherein the apparatus is an integrated circuit.
 14. The apparatus of claim 9, wherein the apparatus is a handset.
 15. An apparatus for predicting a macroblock, the macroblock comprising a plurality of blocks, each block comprising a set of pixel values, the apparatus receiving a prediction mode and a mode uniformity indicator for the macroblock, wherein: if the mode uniformity indictor is true, the apparatus predicts at least two blocks of the macroblock according to the received prediction mode.
 16. The apparatus of claim 15, the apparatus further predicting all blocks of the macroblock according to the received prediction mode if the mode uniformity indicator is true.
 17. The apparatus of claim 15, the mode uniformity indicator being a mode uniformity bit.
 18. The apparatus of claim 15, each block comprising 4×4 pixels, the macroblock comprising 16×16 pixels.
 19. The apparatus of claim 15, wherein the apparatus is an integrated circuit.
 20. The apparatus of claim 15, wherein the apparatus is a handset.
 21. A computer program product for predicting a macroblock, the macroblock comprising a plurality of blocks, each block comprising a set of pixel values, the product comprising: computer-readable medium comprising: code for causing a computer to receive a prediction mode and a mode uniformity indicator for the macroblock; code for causing a computer to, if the mode uniformity indicator is true, predict at least two blocks of the macroblock according to the received prediction mode.
 22. The computer program product of claim 21, comprising code for causing a computer to predict all blocks of the macroblock according to the received prediction mode if the mode uniformity indicator is true.
 23. The computer program product of claim 21, the mode uniformity indicator being a mode uniformity bit.
 24. The computer program product of claim 21, each block comprising 4×4 pixels, the macroblock comprising 16×16 pixels.
 25. The computer program product of claim 21, wherein the product is a handset. 